1. Field of the Invention
This invention relates to a sound attenuator provided in an air passage for reducing the noise generated by a blower, air conditioner, or the like, and including a special porous structure.
2. Description of the Prior Art
A known sound attenuator of the type to which this invention pertains is shown by way of example in FIGS. 1 and 2 and which is disclosed in Japanese Utility Model Publication No. 33898/1985. This device is intended for use in a vacuum cleaner and comprises a cylindrical duct 1, an inner cylinder 2 formed from a nonwoven fabric and having a wall thickness of 0.1 to several millimeters, and a sound-absorbing material 3, such as felt or glass wool, filling the annular space between the duct 1 and the inner cylinder 2. The inner cylinder 2 and the sound-absorbing material 3 cooperate to define a sound absorber. The device is fitted by connectors 4 to an appropriate portion of the air passage of the vacuum cleaner. The inner cylinder 2 has a smooth inner surface formed by treatment with heat or a resin.
This is a typical example of known sound attenuators which can be incorporated in the air passage of a blower, air conditioner, air or vacuum cleaner, or the like for reducing the noise which is thereby generated. In the specific device as hereinabove described, the sound-absorbing material 3 having an indefinite shape is held by and between the duct 1 and the inner cylinder 2 formed from a nonwoven fabric. Sound waves are transmitted through cylinder 2 and are absorbed by material 3. And the inner cylinder 2 has a smoothed inner surface to prevent any fluffing that would otherwise be unavoidable as a drawback of the nonwoven fabric and result in the gathering of dust or dirt by its inner surface, leading eventually to the blocking of the air passage.
The known device has, however, a number of drawbacks. It comprises as many as three components, i.e., the duct 1, the inner cylinder 2 and the sound-absorbing material 3. Its fabrication calls for a fairly complicated process including the step of forming a smooth inner surface on the inner cylinder 2 and the step of incorporating the sound-absorbing material 3 having an indefinite shape. Therefore, the device is considerably expensive to manufacture and yet there is no assurance of all of the products being always of the same reliable quality.
When it is necessary to make a device which can attenuate even sound having a rather low frequency, it is necessary to form the sound-absorbing material 3 with a considerably large thickness, or provide a layer of air between the duct 1 and the sound-absorbing material 3. This necessarily adds to the cost of manufacture and the variation of quality. The sound-absorbing material 3 has a substantially uniform specific density. As it has an indefinite shape, it is difficult to dispose in a way giving it the optimum specific gravity distribution enabling it to exhibit good sound-absorbing properties or to form it into a body having a complicated shape.
Another drawback of the known device is the phenomenon called flanking transmission. Although the device can be elongated to achieve a higher rate of attenuation, its elongation beyond a certain limit brings about a sharp drop in its attenuation rate per unit length, since the noise caused by the propagation of vibration through the sound-absorbing material 3 becomes predominant and is transmitted to the exit of the device without being substantially attenuated. This phenomenon is discussed in detail by William F. Kerka in his paper entitled "Attenuation of Sound in Lined Ducts With and Without Air Flow", ASHRAE JOURNAL, Mar. 1963.